EP-B-0 591 888 discloses a process for working up aqueous dispersions of fluorinated thermoplastics, which comprises using hydrogen ions for substantive replacement of the cations in the aqueous dispersion resulting from the polymerization, compressing the dispersion, if necessary after diluting with water, releasing the compressed dispersion through one or more small aperture(s) so that it coagulates, applying the coagulated phase, if necessary after diluting with water and/or heating, to a filter where it is washed, mechanically dewatering the product, comminuting the same to give a free-flowing product and, if necessary, drying to the desired residual moisture. The work-up preferably takes place continuously.
The compression here may take place in commercially available high-pressure homogenizers. The compressive pressure depends on the apparatus available, and pressures are usually from 200 to 400 bar, appropriately about 300 bar.
In particular for semicrystalline fluorinated thermoplastics, this coagulation process gives excellent results with respect to degree of precipitation and purity of the precipitated product, and for process operating times. However, it is not transferable to fluoroelastomers, since some of the dispersion coagulates during the pumping required for compression, forming a coating on the apparatus, because the coagulate is tacky.
This makes continuous operation of the coagulation apparatus impossible.
Another disadvantage of the known process is that the apparatus required for the compression of dispersions is not everywhere available.
It has now been found that the process mentioned can be transferred to fluoroelastomers if the dispersion is compressed with the aid of a gas and the resultant compressed dispersion is released in a manner known per se through one or more small apertures, and so coagulates.
The invention therefore provides a process for the coagulation of fluoropolymer dispersions, in which the dispersion is compressed by gas pressure and the compressed dispersion is released through one or more small apertures, and so coagulates. Preferred embodiments of this invention are described in more detail below.
Fluoropolymers which may be used are any of the usual highly fluorinated polymers, in particular fluorinated thermoplastics prepared by the known emulsion polymerization process, and especially fluoroelastomers, or else mixtures of these with fluorinated thermoplastics. The crude dispersions may therefore be used directly with polymer contents of from 10 to 45% by weight.
Use may be made of any gas which does not react with the fluoropolymer or with the water, and these include carbon dioxide. Advantageous gases are those whose capability for dissolution in the dispersion under the conditions used is relatively good, since the result is a relatively porous coagulate, and therefore undesirable ancillary substances are relatively easy to remove during the washing which follows. The dissolution of the gas in the dispersion may take place at low pressure, followed by setting of the desired final pressure.
The amount of a gas which enters into the dispersion by way of dissolution under the desired pressure can be influenced by agitation, the mode of feedxe2x80x94for example by bubblingxe2x80x94and the duration of contact. If necessary, one skilled in the art may carry out routine experiments in this regard.
The dissolution of the gases and the application of pressure may take place at from 0 to 100xc2x0 C., preferably at from about 20 to 40xc2x0 C.
The compressive pressure depends on the gas used and may readily be determined by simple exploratory experiments. Pressures are usually from 50 to 400 bar (5 xc2x710xe2x88x926 to 4xc2x710xe2x88x927 Pa). The gas may be compressed using the usual compressors, that is to say the apparatus costs for producing the pressure required are substantially lower than for the process of EP-B-0 591 888.
Gases which may be used are inert gases, such as helium or argon, nitrogen, carbon dioxide, air and fluorinated hydrocarbons, where these do not liquefy under the conditions used. Preference is given to nitrogen, air and carbon dioxide.
The process of the invention can achieve high precipitation rates, and this is important not only with regard to the final yield of polymer but also for the recovery of the fluorinated emulsifier usually used from the wastewater formed in this process and composed of the aqueous filtrate, the treatment of which is made easier at the low resultant polymer concentrations in the filtrate (WO-A-99/62830 and WO-A-99/62858). The polymer content in the wastewater here should be well below 1% by weight.
A great advantage of the process of the invention is that the resultant coagulated polymer can be washed in a manner similar to that of the process of EP-B-0 591 888 on a filter belt, and can be dewatered on a belt press, advantageously at slightly subatmospheric pressure, whereas elastomers usually require complicated apparatus, such as screw systems (U.S. Pat. No. 4,132,845).
According to the invention, as in the process of EP-B-0 591 888, coagulation, washing and dewatering may preferably take place continuously.
The process is particularly advantageous for preparing high-purity polymers, e.g. when coagulating fluoropolymer latices prepared by emulsion polymerization with addition of little or no buffer and/or whose exchangeable ions have been removed, since the xe2x80x9cmechanicalxe2x80x9d coagulation of the invention requires no addition of mineral acids or of salts, another factor advantageous for the recovery of the emulsifier (WO-A-99/62830).
The examples below describe the invention in more detail. Percentages are based on weight.